Build images on CircleCI and upload them to Docker Hub

[humitos]

Following the same pattern we implemented in #179 to build and upload .tar.gz to S3, we should build the Docker image on /release/ubuntu-*/ branches and upload them to Docker Hub. Instead of doing this work manually as I've been doing lately.

[humitos]

Note that we should update our Ubuntu images as well. One of them was built before it was officially released and it's the Development branch:

docs@build-large-i-0dbdef8ccf3db912b(org):~$ docker run --user docs -it --rm readthedocs/build:ubuntu-22.04 /bin/bash
docs@ddf96297b6a3:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu Jammy Jellyfish (development branch)
Release:	22.04
Codename:	jammy
docs@ddf96297b6a3:~$ 
exit
docs@build-large-i-0dbdef8ccf3db912b(org):~$ docker run --user docs -it --rm readthedocs/build:ubuntu-20.04 /bin/bash
docs@ee3a1a942418:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.3 LTS
Release:	20.04
Codename:	focal

[humitos]

I'm removing this task from the roadmap because I think it's more a "Nice to have" currently since we don't have to re-build these images frequently and it won't add too much value to the process. Related: readthedocs/readthedocs.org#11072